1. Field of the Invention
The present invention relates to a method for preparing a green sheet and a method for preparing, from the green sheet obtained according to the foregoing method, a glass ceramic substrate which has a low dielectric constant and which only shows a slight scatter (i.e. variation) in properties from substrate to substrate.
2. Description of the Related Art
Frequencies of signals used are becoming increasingly higher for the purpose of more quickly processing large quantity of information and furthermore, optical signal communication has been adopted as a means for transmitting such high frequency information signals.
In this respect, a multilayered circuit board on which an electronic circuit for processing such high speed or high frequency, is to be formed, must fulfill the requirements such that the delay time .tau. of an electric signal should be as small as possible and that the cross talk between wirings should be very low. In order to obtain substrates which these requirements, the dielectric constant .epsilon. of materials for the substrate must be reduced, as will be seen from the following relation (1): EQU .tau.=.epsilon..sup.1/2 /c (1)
Wherein .epsilon. is the dielectric constant of the material or substrate and c represents the velocity of light.
To substantially reduce the transmission loss, conductive lines, to be formed in a pattern on the substrate, must be formed from a metal having low specific resistance.
Incidentally, it is a matter of course that variations in the electric properties, such as dielectric constant and insulation resistance as well as mechanical properties such as surface roughness and the shrinkage factor, should be very low, from substrate to substrate.
The inventors of this invention have already proposed, as a means for solving these problems, that glass ceramics be used as materials for substrates and that electronic circuits should be formed from copper (Cu).
More specifically, the green sheets mainly comprising alumina (Al.sub.2 O.sub.3 ; .epsilon.=10) should be calcined at a temperature of not less than 1300.degree. C., while the burning temperature for glass ceramic green sheets mainly comprising a mixture of alumina powder and borosilicate glass powder can be reduced down to not more than 1000.degree. C. (see Japanese Unexamined Patent Publication No. 57-995). This permits the formation of a wiring pattern with Cu (having a melting point of 1084.degree. C.).
However, the dielectric constant of such a glass ceramic substrate ranges from 4 to 6 and correspondingly, the reduction of the delay time of signals is not yet sufficient.
Under such circumstances, the inventors of this invention have also proposed the use of hollow silica microspheres to reduce the dielectric constant of substrates (Japanese Patent Application No. 2-252500).
In other words, among the inorganic dielectrics, silica (SiO.sub.2) has the lowest dielectric constant .epsilon. of the order of 3.8. Further, silica powder can be made hollow to thus form a composite dielectric with air. As a result, the dielectric constant can further be reduced as seen from the following relation (2): EQU .epsilon..sub.0 =(.epsilon..sub.1 .multidot..epsilon..sub.2)/(.epsilon..sub.1 v.sub.2 +.epsilon..sub.2 v.sub.1) (2)
Wherein .epsilon..sub.0 is the dielectric constant of the composite dielectric; v.sub.1 is the ratio of the volume occupied by the dielectric having a dielectric constant of .epsilon..sub.1 to the whole volume of the composite dielectric; and v.sub.2 is the ratio of the volume occupied by the dielectric (air) having a dielectric constant of .epsilon..sub.2 to the whole volume of the composite dielectric.
The hollow silica microspheres can be prepared by making use of the phenomenon that in the heat-decomposition of organic silicon-containing compounds such as methoxy silicate [Si(OCH.sub.3).sub.4 ] or ethoxy silicate [Si(OC.sub.2 H.sub.5).sub.4 ], the constituent thereof is separated and forms bubbles. Among the hollow silica microspheres thus prepared, those having a particle size of not less than 100 .mu.m have presently been used as fillers for concrete or additives for light weight molded bodies of resins.
The inventors have also proposed a glass ceramic substrate whose dielectric constant is reduced by the use of hollow silica microspheres having a particle size of not more than 20 .mu.m and preferably 1-10 .mu.m in place of a ceramic which constitutes a glass ceramic substrate (Japanese Unexamined Patent Publication No. 59-111345).
However, in practical applications of such a substrate, the following problems arise:
It was found that when a green sheet was formed in the conventional manner, calcined to give a substrate and the dielectric constant thereof was determined, the dielectric constant varied from substrate to substrate although green sheets prepared from the same slurry were used. Moreover, it was also found that when the green sheets were prepared by the doctor blade method, the dielectric constant of the substrate obtained from the front portion of the slurry was greater than that of the substrate obtained from the middle and end portions of the slurry (see, FIG. 1, FIG. 3 and FIG. 4).
Furthermore, it was likewise found that the green sheet caused crystallization during the burning process, the surface of the resulting substrate was extremely uneven, the thermal expansion coefficient thereof was abnormally increased and thus the resulting substrate was unfavorable for fitting it with an Si semiconductor chip.
Moreover, the inventors have also proposed, as a means for forming a glass ceramic substrate having a low dielectric constant, a method for preparing a glass ceramic substrate which comprises the steps of providing hollow silica microspheres, forming a green sheet from a mixed powder mainly comprising the hollow silica microspheres and borosilicate glass powder and then calcining the green sheet (Japanese Patent Application No. 2-252500).
However, this method suffers from a variety of problems which remains to be solved. For instance, the dielectric constant of the substrates thus prepared greatly varies from substrate to substrate (see FIG. 4) and the silica causes crystallization during the burning process of a green sheet containing the same.
In the preparation of multilayered ceramic substrates, a sheet called "green sheet" comprising a ceramic and a resin is first prepared. However, the hollow silica microspheres, added to a solvent used for the preparation, float on the surface of the resulting slurry (FIG. 1) and finally causes separation, because the specific gravity thereof is smaller than that of the solvent and this correspondingly leads to the formation of non-uniform green sheets (FIG. 3). Moreover, the preparation of a multilayered substrate by the green sheet method suffers from another problem in that a part of the hollow silica microspheres are destroyed because of the pressure of a press applied during lamination. Further, when hollow silica microspheres are incorporated into a resin for the purpose of reducing the weight or dielectric constant thereof, they preferentially distribute in the upper portion of the resin matrix before the solidification thereof because of the difference in the specific gravity between the hollow silica microspheres and the resin and hence it is likewise difficult to uniformly distribute the hollow silica microspheres in this case. For this reason, the shrinkage factor of the portion rich in the hollow silica microspheres after heating is high, while that of the portion deficient in the hollow silica microspheres is low. This becomes a cause of deformation, warps or peeling in case of films, upon heating.